In slit lamp microscopes investigating eye diseases, it has already been proposed to provide uniform illumination around the illuminated slit area so that the position of the slit area can readily be determined with respect to other portions of the eye being investigated. Most conventionally, there has been provided, in addition to the light source for the slit illumination, a separate light source for the auxiliary uniform illumination. However, the arrangement has been found disadvantageous in that it requires two separate light sources which, as a matter of course, increases the manufacturing cost and requires troublesome handling of the components.
In order to eliminate the above disadvantage, there has been proposed in Japanese utility model publication Sho 47-27558 the use of a light diffusing plate having a transparent slit area. However, this proposal has also been found to be disadvantageous in that only a limited bundle of light is allowed to pass through the transparent slit area and therefore the brightness of the slit illumination is decreased.